1. Field of the Invention
The present invention relates to a differential amplification circuit which performs differential amplification of complementary input signals and performs level shifting after the differential amplification, a semiconductor integrated circuit provided with this circuit and a method of noise removal.
2. Description of the Related Art
It is necessary to remove noise to a sufficient degree in a semiconductor integrated circuit provided with a high precision analog circuit. In particular, a semiconductor integrated circuit which includes both an analog circuit and a digital circuit, i.e., a PLL circuit that includes a frequency divider circuit, for instance, noise is communicated from the digital circuit to the analog circuit. Therefore, it is important to remove the noise in the analog circuit. The importance of noise removal increases as the frequency in the digital circuit increases to achieve higher speed.
FIG. 8 shows a schematic structure of a semiconductor integrated circuit. An amplifying circuit 10, a digital circuit 11 and an analog circuit 12 operate under a source voltage VCC supplied from the outside. The analog circuit 12 may be a voltage controlled oscillator, for instance, and the output VAA from the amplifying circuit 10 is supplied to the analog circuit 12 as an input signal and a source voltage. Since noise is communicated to the analog circuit 12 via the wiring of the input Vaa and the source voltage VCC of the amplifying circuit 10, the amplifying circuit 10 includes a noise filter.
However, increasing the capacity of the noise filter in order to remove the noise to a sufficient degree results in poor operating characteristics, decreasing response speed in the amplifying circuit 10.